Re: Convergence as a cue to depth perception

[P3D Gregory J. Wageman <gjw@xxxxxxxxxx>]

John R wonders:

>If you see an object (say 100-300 feet away) with no closer or more distant
>cues (for example against a clear blue sky), does it become extremely difficult
>to estimate the distance? And if a reference is added at "infinity" (for
>instance the moon or a cloud), how much does that help in estimating the
>distance?

I can supply a possibly relevent anecdote here.

A while back I travelled to Amarillo, Texas on business.  Behind the
motel where I stayed was rangeland.  There were some cows in "the distance",
and beyond that a string of power poles of the TxT type (crossbars atop
poles, the poles being interconnected by crossbracing) going out to the
horizon.  There were no other objects to give a sense of scale.

Since I'm not a "flatlander", and northwest Texas is *extremely* flat,
my brain is not used to being able to see to the horizon on land, as
was the case here.  Consequently when viewing the above scene, I had
the disconcerting impression that the cows were *huge*, like 15-20 feet
tall.  In fact, at first I rationalized that they were billboards of some
sort, possibly advertising a local steak house!

My depth perception completely failed to convey the relative distances
between myself, the cows and the power poles.  The powerlines were
actually *miles* away, and the cows relatively close (~300 feet).

Because my brain is unaccustomed to seeing that far on land, I believe
it decided that the cows must be huge for them to appear their size
relative to the poles, which had to be "closer" than they actually were.
The feeling of not being able to "compute" the scene was *very*
disconcerting, almost disorienting.

So to answer the question, I believe even WITH a more distant cue, it
is very hard to estimate distances over a few hundred feet away.

Any sailors on the list?  Certainly this sort of thing must occur at
sea, where visibility often extends to the horizon with no depth cues.
How hard is it to correctly judge distances in such a case?

	-Greg W.


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